1.5 SECONDARY DISTRIBUTION SYSTEMS. The secondary distribution system is that
portion of the network between the primary feeders and utilization equipment. The secondary
system consists of step-down transformers and secondary circuits at utilization voltage levels.
Residential secondary systems are predominantly single-phase, but commercial and industrial
systems generally use three-phase power.
1.5.1 Secondary Voltage Levels. The voltage levels for a particular secondary system are
determined by the loads to be served. The utilization voltages are generally in the range of 120 to
600 V. Standard nominal system voltages are:
In residential and rural areas the nominal supply is a 120/240 V, single-phase, three-wire
grounded system. If three-phase power is required in these areas, the systems are normally
208Y/120 V or less commonly 240/120 V. In commercial or industrial areas, where motor loads
are predominant, the common three-phase system voltages are 208Y/120 V and 480Y/277 V.
The preferred utilization voltage for industrial plants, however, is 480Y/277 V. Three-phase
power and other 480 V loads are connected directly to the system at 480 V and fluorescent
lighting is connected phase to neutral at 277 V. Small dry-type transformers, rated
480-208Y/120 or 480-120/240 V, are used to provide 120 V single-phase for convenience outlets
and to provide 208 V single- and three-phase for small tools and other machinery.
1.5.2 Types of Systems. Various circuit arrangements are available for secondary power
distribution. The basic circuits are: simple radial system, expanded radial system, primary
selective system, primary loop system, secondary selective system, and secondary spot network.
18.104.22.168 Conventional Simple-Radial Distribution System. In the simple-radial system